The invention refers to an apparatus for measuring forces by means of at least one roller bearing, of which the outer and/or inner ring, at different points of the bearing, are provided with at least two strain gauges which serve to record the forces to be measured, and of which the resistance variations are amplified and forwarded for their use.
An apparatus of the above mentioned kind is well known, which measures strains generated in the outer rings as a result of the load applied by the roll bodies. In this apparatus, the strains generated on the stationary outer ring of the bearing, as a result of being rolled over by the loaded roll bodies, are measured by means of gauges provided in four areas of the bearing. When a roll body lies within the measuring range, the strain is at its maximum, and when the measuring range lies between the roll bodies, the strain is close to zero. The rolling over of a given measuring range in the outer ring thus occurs in the form of a harmoniously alternating strain load. The number of strain peaks per time unit corresponds to the number of roll bodies rolling over the measuring range within the same time unit. To mount the roller bearing in a conventional manner, the strain gauges arranged in the peripheral area of the outer ring, are placed in notches or small cuts. The measuring grids of the strain gauges run perpendicular to the roller bearing axis. The strains of the strain gauges are recorded, amplified and made visible in an oscillograph or a recorder, of which the cutoff frequency has to be at least twice as high as the roll-over frequency. The roll-over frequency results from the speed of the shaft n, the diameter of the roll body D, the pitch diameter d, the number of roll bodies z and the angle of contact .alpha. in the bearing (Instrumens & Control Systems 1964, 132 et al.). With this well-known apparatus it is not possible to accurately record the load on the bearing for a long period of time and in such a manner that the measuring signal, which is proportional to the force, can subsequently be used for process controlling purposes. Indeed, the load acting between the roller matings is not solely dependent on the forces attacking from the outside. In prestressed roller bearings, even in unloaded contition, forces will already be active between the roll bodies and the bearing rings. On the other hand, in bearings with play, the portion of the total load carried by a roll body, will depend on the play of the bearing. However, the prestressing or play of a roller bearing are quantities which change during the operation. In the operating condition there normally occurs a temperature drop from the inner to the outer ring of the roller bearing, following which the play of the bearing will decrease or the initial stress will increase. Furthermore, the measured alternating strain in the well-known apparatus, because of unavoidable formal defects in the bearing, is not constant. Formal defects in the inner ring and differences in the diameter of the roll bodies mean that the amplitudes of the alternating strain will fluctuate around an average value. In addition, due to hydrodynamic phenomena in the lubricant, especially at high speeds, strains can occur in the outer ring which are not generated by external forces. Beyond this, imbalances in the amplitude of the alternating signal may superpose themselves in the form of beats. An evaluation of the measuring signal in prestressed bearings is further complicated in that the ascent of the strain already generated by the initial stress, will be so little affected by an external force that an increase in the amplitude of the alternating strain will hardly be visible.
The well-known above described apparatus has therefore a number of serious drawbacks. The evaluation requires recording instruments of which the cutoff frequency has to be at least twice as high as the rolling-over frequency. The evaluation of the measuring results is made difficult, if not impossible, in that the strains generated by external forces are substantially lower than the strains caused by the initial stress in the bearing. The prestressing or the play of the bearing will be modified during the operation through variable heating of the inner-outer-ring and the roll bodies. This in turn originates changes in the signals which are not traceable to external forces. Strain changes not resulting from external loads, especially at high speeds, are generated by hydrodynamic phenomena in the lubricant. Formal defects in the bearing, imbalances and centrifugal forces cause the alternating strain amplitude to fluctuate around an average value which is difficult to determine. Finally, the so obtained signal is not suited for further use in a process-controlling action.
A force-measuring roller bearing is also well known in which a static measuring device is used. In this bearing the measuring grid length is selected in such a manner that it represents a multiple of the roll body distance. However, since the measuring grid is not extended in its full length, whereas the recorded strains are very small, it is necessary to select amplifiers with a very high amplifying factor. Owing to the well-known drawbacks of d.c. voltage amplification, this will make the measurements very unreliable.
Both well-known apparatuses do not allow to measure a load applied to the bearing by external forces for any long period of time.